costs to buld:
from alro
6' steel flat (1/2" x 2.5" x 6') - 28.50 +tax
6' alum flat (1/2" x 2.5" x 6') - 40 +tax

rollers - $30ea (w.marine / less expensive also aval for less localy) - $120
(old rollers show cracks)

bolts, washers, fasteners (at least) $5ea +tax - 20 (roller shafts and ends seem much more expensive with a quick google search)

$181.90 (inc tax) before welding labor

The one's online look galvanized or painted, or powdercoatd as well